Electrolysis of salt solution

ABSTRACT

AN AQUEOUS SOLUTION OF ALKALI METAL HALIDE OR HYDROCHLORIC ACID IS ELECTROLYZED IN AN ELECTROLYTIC CELL CONSISTING OF AT LEAST ONE UNIT CELL WHICH COMPRISES A CATHODE, AN ANODE, A CATION EXCHANGE MEMBRANE, AND A NEUTRAL DIAPHRAGM HAVING A WATER PERMEABILITY OF NOT MORE THAN 5 CC/MIN. CM.2 UNDER A PRESSURE DIFFERENCE OF 1 KG./CM.2, WHERE THE CATION EXCHANGE MEMBRANE AND THE NEUTRAL DIAPHRAGM ARE JUXTAPOSED AT A DISTANCE BETWEEN THE CATHODE AND THE ANODE TO FORM AN ANODE COMPARMENT BETWEEN THE ANODE AND THE NEUTRAL DIAPHRAGM, AN INTERMEDIATE COMPARTMENT BETWEEN THE NEUTRAL DIAPHRAGM AND THE CATION EXCHANGE MEMBRANE, AND A CATHODE COMPARTMENT BETWEEN THE CATION EXCHANGE MEMBRANE AND THE CATHODE, BY PASSING EACH COMPARTMENT SOLUTION AT A FLOW VELOCITY OF AT LEAST 3 CM./SEC., WHILE KEEPING AN INSIDE PRESSURE OF THE INTERMEDIATE COMPARTMENT HIGHER THAN THAT OF THE ANODE COMPARTMENT.

2 Sheets-Sheet '1 MITSUO YOSHIDA ETAL ELECTROLYSIS OF SALT SOLUTIONApril 4, 1972 Filed Feb. 5,

April 1972 MITSUO YOSHIDA ETAL 3,654,104

ELECTROLYSIS OF SALT SOL' JTION Filed Feb. 5 1970 2 Sheets-Sheet 2 E) 45it in J i J 6 8 /0 W l/UOK/ 7') //v CELL (Cm/56C) United States Patent O3,654,104 ELECTROLYSIS F SALT SOLUTION Mitsuo Yoshida, Hisao Kai, andTetsuo Yamane, Nobeoka-shi, Japan, assignors to Asahi Kasei KogyoKabushiki Kaisha, Osaka, Japan Filed Feb. 5, 1970, Ser. No. 8,325 Claimspriority, application Japan, Feb. 15, 1969, 44/10,766 Int. Cl. B01d59/42; BOlk 1/00; C01d 1/06 US. Cl. 204-98 3 Claims ABSTRACT OF THEDISCLOSURE An aqueous solution of alkali metal halide or hydrochloricacid is electrolyzed in an electrolytic cell consisting of at least oneunit cell which comprises a cathode, an anode, a cation exchangemembrane, and a neutral diaphragm having a water permeability of notmore than 5 cc./ min. cm. under a pressure difference of 1 kg./cm. wherethe cation exchange membrane and the neutral diaphragm are juxtaposed ata distance between the cathode and the anode to form an anodecompartment between the anode and the neutral diaphragm, an intermediatecompartment between the neutral diaphragm and the cation exchangemembrane, and a cathode compartment between the cation exchange membraneand the cathode, by passing each compartment solution at a flow velocityof at least 3 cm./sec., While keeping an inside pressure of theintermediate compartment higher than that of the anode compartment.

This invention relates to a process for electrolysis of an aqueouselectrolyte solution, and more particularly, it relates to a process forelectrolyzing an aqueous solution of alkali metal halide or hydrochloricacid in a three-compartment electrolytic cell containing a neutraldiaphragm and a cation exchange membrane.

Several proposals have been made concerning processes for electrolysisof an aqueous electrolyte solution by utilizing an ion exchangemembrane. It is, however, very difiicult to employ them commercially,because they have some serious defects from an industrial point of view.

In US. Pat. Nos. 3,017,338 and 3,135,673, for example, there aredisclosed methods for obtaining caustic soda by electrolyzing an aqueoussodium chloride solution in a three-compartment electrolytic cellcontaining a cation exchange membrane and a highly water-permeableneutral diaphragm. These prior methods have the inevitable drawbacksthat caustic soda of low grade, i.e. containing impurities, is formed asa by-product; since the whole amount of feed electrolyte solution mustbe passed to the corresponding compartment through the neutraldiaphragm, this is liable to cause clogging of the neutral diaphragm andso stationary, stable operation for a long period is hardly expected;and polarization of the cation exchange membrane is apt to occur, whichresults in an increase in the electric voltage required for theelectrolysis.

It is an object of this invention to overcome the abovesaid drawbacks ofthe heretofore-proposed methods for electrolyzing the aqueouselectrolyte solution by using an ion exchange membrane and a neutraldiaphragm. That is, the object of the present invention is to provide amethod by which the electrolysis of alkali metal halide or hydrochloricacid may be carried out while maintaining the conditions at high currentdensity under low voltages with high stability for a long period.

It has now been found the above object is attained by carrying out theelectrolysis in a three-compartment electrolytic cell which contains aneutral diaphragm having very low water-permeability and a cationexchange mem- "ice brane, while passing the electrolyte solution underrestricted condition.

The electrolytic cell employed in this invention consists of at leastone unit cell which comprises a cathode, an anode, a cation exchangemembrane and a neutral diaphragm having a very low water permeability,Where the cation exchange membrane and the neutral diaphragm arejuxtaposed at a distance between the cathode and the anode so that thecation exchange membrane and the neutral diaphragm may be at the cathodeside and at the anode side respectively, to form an anode compartmentdefined by the anode and the neutral diaphragm, an intermediatecompartment defined by the neutral diaphragm and the cation exchangemembrane and a cathode compartment defined by the cation exchangemembrane and the cathode. According to this invention, an aqueouselectrolyte solution is passed through the anode compartment and theintermediate compartment and a cathode solution through the cathodecompartment at flow velocities of at least cm./ sec. respectively, whilekeeping an inside pressure of the intermediate compartment higher thanan inside pressure of the anode compartment.

The invention is further explained by referring to the accompanyingdrawings.

FIG. 1 shows a sectioned schematic view of the electro lytic cell usedin this invention and a flow diagram showing an embodiment of thepresent invention.

FIG. 2 is the graph showing a relation between a flow velocity within anelectrolytic cell and voltage of the cell of Example 3.

In FIG. 1, an electrolytic cell 19 consists of an anode 1, a cathode 2,a neutral diaphragm 3, and a cation exchange membrane 4, where theneutral diaphragm 3 and the cation exchange membrane 4 are juxtaposed ata distance from about 1 to 10 mm. between the anode 1 and the cathode 2so that the neutral diaphragm may be at the anode side and the cationexchange membrane at the cathode side, whereby there are formed an anodecompartment 9 by the anode 1 and the neutral diaphragm 3, anintermediate compartment 8 by the neutral diaphragm 3 and the cationexchange membrane 4 and a cathode compartment 13 by the cation exchangemembrane 4 and the cathode 2. The thickness of each compartment isusually several millimeters. In FIG. 1, an electrolytic cell consistingonly of one unit cell is shown for simplicity of illustration, but apractical electrolytic cell consists of a stack of a plurality of suchunit cells.

As the material for the anode 1, there may be used the anode materialsusually used in the conventional electro lytic cell, for example,graphite, platinum, and titanium or tantalum plated with a noble metalsuch as platinum or rhodium.

As the material for the cathode 2, there may be employed the cathodematerials usually used in the conventional electrolytic cell, forexample, iron, stainless steel, and nickel.

In the present invention, it is necessary that the neutral diaphragm 3have a water permeability as low as possible, and it is desirable thatthe water permeability under a pressure difference of 1 kg./cm. be notmore than 5 cc./min. cm. preferably not more than 2 cc./min. cm. Theneutral diaphragm having such properties may be easily manufactured byknown methods, from an acidresistant and chlorine-resistant material,e.g. blue asbestos, polytetrafluoroethylene (Teflon) and polyesters, andit can be reinforced, if necessary, by surface-treatment, incorporationof a reticular material of polyvinylidene chloride (Saran), polyester orpolyethylene as a core or along the surface of the diaphragm. Forexample, a neutral diaphragm of blue asbestos is obtained by dispersinga binder such as a latex of polyvinylidene chloride (Saran), into anaqueous slurry of blue'asbestos, then subjecting the slurry to make adiaphragm and pressing the same, and if necessary, applying a suitablematerial such as divinylbenzene onto the surface of the resultantdiaphragm; or thediaphragm of plastics may be obtained by mixing theplastics such as polytetrafluoroethylene, polyvinylidene chloride andpolyethylene terephthalate with a plasticizer, then molding the mixtureinto film, and extracting the plasticizer out of the film.

As the cation exchange membrane 4, there may be employed any one whichis prepared according to the Wellknown method, but among which there arepreferably used those having the properties of a low alkali diffusionfrom the cathode compartment 13 to the intermediate compartment 8 and agood cation transport number.

In this invention the feed electrolyte solutions are alkali metalhalides such as sodium chloride and potassium chloride, and hydrochloricacid. When the alkali metal halide is fed the products obtained areessentially a solution of the corresponding hydroxide, gaseous hydrogenanad chlorine. When hydrochloric acid is fed the products areessentially gaseous hydrogen and chlorine.

Now, the present electrolytic method will be explained for the case whenthe feed solution is an alkali metal halide, referring also to FIG. 1wherein each compartment solution is recycled between the tanks forrecycle and the electrolytic cell.

A11 aqueous salt solution 5 to be electrolyzed is supplied to a tank 6for recycle of an intermediate compartment solution. The intermediatecompartment solution is supplied from the tank 6 to the intermediatecompartment 8 by means of a pump 17. Although a very small portion ofthe intermediate compartment solution sometimes passes from theintermediate compartment 8 to the anode compartment 9 through theneutral diaphragm 3, almost all of the intermediate compartment solutionleaves the intermediate compartment and returns to the tank 6. A portionof the thus returned solution is supplied to a tank 7 for recycle of theanode compartment solution. The anode compartment solution is suppliedfrom the tank 7 to the anode compartment 9 by means of a pump 16, andreturned again to the same tank 7, after the gas generated at the anodehas been separated. A portion of the returned anode compartment solutionleaving the electrolytic cell is, after the residual anode gas beingremoved therefrom at an anode gas-removing apparatus 10, then admixedwith the raw material salt and water, and supplied to the tank 6 as asolution 5 through an apparatus 11 for purifying an aqueous saltsolution.

On the other hand, the cathode compartment solution I is likewiserecycled between a tank 12 for recycle of the cathode compartmentsolution and the cathode compartment 13 by means of a pump 18, and afterthe gas generated at the cathode has been separated, a portion of thereturned cathode compartment solution leaving the electrolytic cell istaken out of the electrolytic system as the product. To the tank 12water 15 is supplied separately.

In the present invention, it is necessary that the pressure 'of theintermediate compartment 8 be higher than that of the anode compartment9 within the electrolytic cell at said solution recycle. Furthermore, itisimportant for the operation of low voltage, as shown in the exampleswhich follow, that the flow velocity of each compartment solution be atleast 3 cm./ sec. By doing so, the transference of some amount ofsolution from the intermediate compartment-S to the anode compartment 9through the neutral diaphragm 3 mayoccur.

If a neutral diaphragm having a high water permeability isused, a largeamount of the intermediate compartment solution is passed to the anodecompartment 9 at the lower part of the electrolytic cell, but it ishardly passed at the upper part of the electrolytic cell. Therefore, aportion of the anode compartment solution flows back to the intermediatecompartment 8 at the upper part of r 4 the electrolytic cell, and itbecomes impossible to effect operation worth the provision of theneutral diaphragm 3. That is, it is impossible to efiect the operationof supplying an aqueous salt solution free of chlorine gas to thesurface of the cation exchange membrane 4. Furthermore, there is almostno How of the intermediate compartment solution at the surface of thecation exchange membrane 4, resulting in increase in voltage.

On the other hand, when a neutral diaphragmfi having a very low waterpermeability is used according to this invention, substantially notransference of the solution from the intermediate compartment 8 to theanode compartment 9 through the neutral diaphragm 3 occurs; furthermore,substantially no flow velocity is changed at both upper and lower partsof the intermediate compartment a; and a very small amount of theintermediate compartment solution is only transferred to the anodecompartment equally on the entire surface of the neutral diaphragm 3.Thus, according to this invention, clogging of the neutral diaphragm 3and contamination of the anode compartment solution into theintermediate compartment never occur; the properties of the neutraldiaphragm 3 and the cation exchange membrane 4 are not deteriorated evenwhen they are used for a prolonged period of time.

In the present invention, the flow velocity of each compartment solutionWithin the electrolytic cell is adjusted to at least 3 cm./sec., wherebythe distribution of solution flow may be improved, a fear ofpolarization of the cation exchange membrane 4 may be completelyeliminated, and disengagement of the gases at the anode 1 and cathode 2from the electrode surfaces'may be accelerated to keep a stableoperation under a low voltage.

In the foregoing, the explanation has been made as to the case that eachcompartment solution is recycled between the electrolytic cell and thetanks, but it is possible to eliminate the tanks in the embodiment ofthe present invention.

The process of this invention relating to the conversion of alkali metalhalide and hydrochloric acid into their corresponding halogen, metalhydroxide and hydrogen are illustrated by the following examples whichare not to be construed as limiting.

EXAMPLE 1 Electrolysis of sodium chloride by using a neutral diaphragmhaving a high water permeability which is not iucluded in the scope ofthis invention.

There was used an electrolytic cell consisting of: as an anode aplatinum-plated titanium electrode; as a cathode a stainless steelelectrode; as a neutral diaphragm a polyester fabric having a waterpermeability as high as 20 cc./min. cm. under a pressure difference of 1kg./ cm. and as a cation exchange membranea homogeneous membraneprepared by copolymerizing the phenyl ester of vinyl sulfonic acid,styrene and divinylbenzene as its principal constituents and hydrolyzingthe thus obtained copolymer; an available area for current passage ofthe electrodes, the membranes and the diaphragm being 500 cm. (5 cm. xcm.); an interelectrode distance being. 8 mm.; the thicknesses of theanode compartment, the intermediate compartment and the cathodecompartment being 2.5 mm., 2.0 mm. and 2.8 mm. respectively; and thethicknesses of the neutral diaphragm and the cation exchange membranebeing 0.5 mm. and 0.2 mm. respectively. An aqueous 5 N 'NaCl solutionwas recycled as both anode compartment solution and intermediatesolution, and an aqueous 5 N NaOH solution was recycled as the cathodecompartment solution. The solution temperature and the D.C. electriccurrent were 60 C. and 75 a. (15 a./dm.'-), respectively. The flowvelocity of each compartment solution was 4 crn./sec. at the inlet ofthe electro lytic cell and the pressure in the intermediate compartmentwas made higher by approx. 0.10 kg./cm. than the pressure in the anodecompartment. By passing an electric current, most portions of theintermediate compartment solution were transferred to the anodecompartment through the neutral diaphragm 'at the lower part of theelectrolytic cell, and on the contrary, the anode compartment solutionwas transferred to the intermediate compartment at the upper part of theelectrolytic cell. The returned intermediate compartment solution onlycorresponded to a flow velocity of 0.5 cm./ sec. Through the cellvoltage was 4.58 v., the alkali current efficiency was 92%.

Then, further operation was carried out by adjusting the flow velocitiesof the anode compartment solution and the intermediate compartmentsolution to 1 cm./sec. and 7 cm./sec. at the inlet of the electrolyticcell respectively, and could be effected without any transference of theanode compartment solution to the intermediate compartment in theelectrolytic cell, but the returned intermediate compartment solutiononly corresponded to a flow velocity of 1.0 cm./sec. The cell voltagewas 4.40 v. and the alkali current efiiciency was still 92%. However, inthat case, the purified salt solution could not be supplied in time tothe tank 6 for recycle of the intermediate compartment solution, and aportion of the returned anode compartment solution had to be supplied tothe tank 6 for recycle of the intermediate compartment solution,resulting in contamination of chlorine gas into the intermediatecompartment solution.

The contamination of the chlorine gas into the intermediate compartmentsolution caused the cation exchange membrane to deteriorate, and thevoltage was increased by about 0.2 v. after a continuous operation of100 hours, and the alkali current efiiciency was reduced by 4%, that is,to 88%.

EXAMPLE 2 Electrolysis of sodium chloride by using a neutral diaphragmhaving a low water permeability.

Electric current was passed through the same electrolytic cell under thesame conditions as in Example 1, except that there was used a neutraldiaphragm of blue asbestos sheet whose surface had been treated withdivinylbenzene for reducing its porosity and reinforced with a Saran netalong the surface, having a water permeability as high as cc./min. cm.under a pressure difference of 1 kg./cm. and a flow velocity of eachcompartment solution was adjusted to 4 cm./sec. at the inlet of theelectrolytic cell. As a result, the intermediate compartment solutiontransferred to the anode compartment through the neutral diaphragmamounted to about 50 cc./min., and the returned intermediate compartmentsolution corresponded to a flow velocity of about 3.1 cm./sec. Nocontamination of chlorine gas was brought about at all. By passingcontinuously an electric current for 100 hours, the cell voltage was4.20 volt and stable. No deterioration of the cation exchange membranewas brought about, and the alkali current efiiciency was 93%.

EXAMPLE 3 The relation between the flow velocity in each compartment atthe outlet and the voltage was determined in the same electrolytic cellunder the same conditions as in Example 2, and one of the results isshown in FIG. 2. The electrolytic conditions are given below:

Anode and intermediate compartment solutions: 5 N

NaCl,

Cathode compartment solution: 5 N NaOH, solution,

Temperature: 60 C., and

Electric current: 75 a. a./dm.

EXAMPLE 4 An electrolytic cell consisting of 7 unit cells, each unitcell having an available area of 70 dm. (70 cm. x100 cm.) for currentpassage and all the unit cells being stacked in a filter press was used,where there was used as an anode a platinum-plated titanium electrode,as a cathode an iron electrode, as a neutral diaphragm a blue asbestossheet incorporated with a polyester fabric as a core material andsurface-treated with dinylbenzene for reducing its porosity, and as acation exchange membrane a membrane of the vinyl sulfonicacid-divinylbenzene type. The interelectrode distance was 8 mm. Therewere used an aqueous ca. 5 N NaCl solution as both anode andintermediate compartment solutions, an aqueous 5 N NaOH solution as acathode compartment solution, and a saturated NaCl solution as apurified aqueous salt solution to be supplied to the tank 6 for recycleof the intermediate compartment solution. Operation was carried out at asolution temperature of 70 C. with a current of 7350 a. (15 a./dm. at aflow rate of each compartment 'solution of about 40 lit./min. and a flowvelocity of about 7 cm./sec. within the electrolytic cell, while keepingthe pressure within the intermediate compartment higher by about 0.1kg./cm. than that within the anode compartment. No contamination ofchlorine gas into the intermediate compartment solution was broughtabout, and the amount of the intermediate compartment solutiontransferred to the anode compartment through the neutral diaphragm wasabou 5 lit/min. and was not changed almost at all. The cell voltage was3.80 v. and the alkali current efliciency was 94.5%. No deterioration ofthe cation exchange membrane was brought about, and no change wasobserved in the water permeability and electnc resistance of the neutraldiaphragm. Stable operation could be continued for 93 days.

EXAMPLE 5 In the same apparatus as employed in Example I, there wereassembled the cation exchange membrane, which was prepared bypolymerizing acrylic acid, divinylbenzene and styrene into a film shapeand then saponifying the polymerizate with caustic soda, and the neutralmembrane which was prepared by a paper-making process from an aqueousslurry of unravelled blue asbestos contaming Saran latex as a bindermaterial and then followed by being pressed under a pressure of kg./cm.which had a water permeability of 2 cc./min. cm. under a pressurediiference of 1 kg./cm. A saturated aqueous solution of potassiumchloride was supplied into the intermediate and anode compartments, andan aqueous solution of 5 N-KOH was supplied into the cathodecompartment, at the linear flow velocity of 4 cm./sec. at the inlet partof each compartment. Electrolysis was carried out under the conditionthat the solution temperature and the DC. electric current were 60 C.and 75 a. (15 a./dm. respectively.

The pressure within the intermediate compartment was made higher byapprox. 0.1 kg./cm. than that within the anode compartment.

As a result, the intermediate compartment solution transferred to theanode compartment through the neutral diaphragm amounted to approx. 18cc./min., the returned intermediate compartment solution corresponded toa flow velocity of approx. 3.7 cm./sec., and no contamination ofchlorine gas into the intermediate compartment solution was observed. Byeffecting a continuous electrolysis for 25 days the cell voltage showed3.8 volts and stable, no deterioration of the cation exchange membraneand neutral diaphragm was found, and the alkali efliciency was 94%EXAMPLE 6 Electrloysis of an aqueous hydrochloric acid solution wascarried out by use of the same electrolytic cell, ion exchange membraneand neutral diaphragm. Aqueous 10 N HCl solutions were fed into both theanode compartment and the intermediate compartment, and an aqueous 5 NHCl solution was fed into the cathode compartment, at the linear flowvelocity of 4 cm./sec. at the inlet part of each compartment, at thetemperature of 60 C. with a electric current density of 15 amp./dm. Thepressure within the intermediate compartment was made higher by approx.0.05 kg./c1n. than that within the anode compartment. As the result, aintermediate compartment solution transferred to the anode compartmentthrough the neutral diaphragm amounted to approx. 22 cc./min., thereturned intermediate compartment solution corresponded to a flowvelocity of approx. 3.6 cm./sec. and no contamination of chlorine gasinto the intermediate solution was observed. By effecting the continuouselectrolysis for 100 hours a cell voltage showed 3.20 volts and stable,and substantially no deterioration of the cation exchange membrane andthe neutral diaphragm was observed.

What we claim is:

1. In a method for electrolyzing an aqueous solution of alkali metalhalide or hydrochloric acid by using an electrolytic cell consisting ofat least one unit cell which comprises a cathode, an anode, a cationexchange membrane and a neutral diaphragm, where the cation exchangemembrane and the neutral diaphragm are juxtaposed at a distance betweenthe cathode and the anode so that the cation exchange membrane and theneutral diaphragm may be at the cathode side and at the anode siderespectively, to form an anode compartment defined by the anode and theneutral diaphragm, an intermediate compartment defined by the neutraldiaphragm and the cation exchange membrane, and a cathode compartmentdefined by the cation exchange membrane and the cathode, tne improvementwhich comprises passing the aqueous solution to be electrolyzed throughthe anode compartment and the intermediate compartment and a cathodesolution through the cathode compartment at How velocities of'at least 3cm./sec., respectively, while using a neutral diaphragm having a waterpermeability of not more than 5 cc./min. cm. under a pressure difierenceof 1 kg./cm. and keeping an inside pressure of the intermediatecompartment higher than that of the anode compartment whereby a part ofthe intermediate solution is introduced into the anode compartmentthrough said neutral diaphragm.

2. The improvement according to claim 1, wherein the aqueous solution tobe electrolyzed is an aqueous alkali metal halide solution.

3. The improvement according to claim 1, wherein the aqueous solution tobe electrolyzed is an aqueous hydrochloric acid solution.

References Cited UNITED STATES PATENTS 3,135,673 6/1964 Tirrell et al.204--98 3,222,267 12/1965 Tirrell et al. 20498 3,458,411 7/1969 Grotheeret al. 204-128 3,438,879 4/1969 Kircher et al. 204-98 X 3,524,801 8/1970Parsi 20498 HOWARD S. WILLIAMS, Primary Examiner D. R. VALENTINE,Assistant Examiner US. Cl. X.R. 204-128, 129, 180 P

